• Clinical History
• Electron Microscopy
  (low magification)
• Electron Microscopy
  (high magification)
• Diagnosis and Comment

Diagnosis

Metastatic malignant melanoma, primary not identified - Right axillary mass.

Comments

This case illustrates the utility of electron microscopy in rapidly establishing a definitive diagnosis through the identification of a structurally specific cytoplasmic organelle. Although not clinically suspected, but certainly included in the differential diagnosis histologically, a diagnosis of metastatic melanoma was obvious ultrastructurally and confirmed immunocytochemically.

Malignant melanoma, with or without a known primary, is a frequent consideration in metastases to lymph node, lung, mediastinum, brain, liver, and other sites, especially when the lesion is composed of relatively large, epithelioid appearing tumor cells. Even when the patient has a history of a resected malignant melanoma of skin, tumor cells within the metastasis may contain little or no melanin pigment and the diagnosis is then difficult to confirm by light microscopy. One of the most effective methods to establish a definite diagnosis in this circumstance is electron microscopy. The identification of typical melanosomes, or even aberrant melanosomes (perhaps a topic for a future on-line conference), at stages prior to the synthesis of melanin pigment, allow confirmation of a suspected metastatic melanoma. If melanosomes are absent, however, other ultrastructural features may enable diagnoses such as primary or metastatic squamous carcinoma and adenocarcinoma, germ cell tumors and various sarcomas, as well as diseases such as nonHodgkin’s lymphoma and Hodgkin’s disease.